Figure 1.

Rule learning-induced enhancement of spontaneous inhibitory events. A, Schematic description of the four-arm maze. Protocols for trained and pseudo-trained rats are similar: an electronic “start” command randomly opens two of eight valves (V), releasing a positive-cue odor (P) into one of the arms and a negative-cue odor (N) into another. Eight seconds later, the two corresponding guillotine doors (D) are lifted to allow the rat to enter the selected arms. Upon reaching the far end of an arm (90 cm long), the rat body interrupts an infrared beam (I, arrow) and a drop of drinking water is released from a water hose (W) into a small drinking well (for a trained rat, only if the arm contains the positive-cue odor, for pseudo-trained rat, randomly). A trial ends when the rat interrupts a beam, or in 10 s, if no beam is interrupted. A fan is operated for 15 s between trials, to remove odors. Each rat had 20 trials per day. B, Spontaneous inhibitory synaptic events recorded in a neuron from a trained rat at a holding potential of −60 mV, in the presence of 1 μm TTX, 20 μm DNQX, and 50 μm APV. Spontaneous activity was totally abolished in the presence of the GABA_A blocker BMI (20 μm). C, Average amplitude of spontaneous events in neurons from trained rats is significantly higher than in neurons from the two control groups. The average amplitude was calculated for each neuron from all spontaneous events. Values (mean ± SE) represent the average of all cells in each group (***p < 0.001). Data taken from 26 trained, 25 naive, and 19 pseudo-trained rats. D, Cumulative frequency distributions of event amplitudes of the three groups. Each point represents the average event amplitude in a neuron. The average sIPSC amplitude appears to increase in most neurons in the trained group.